Some pyrrolobenzodiazepines (PBDs) have the ability to recognise and bond to specific sequences of DNA; the preferred sequence is PuGPu. The first PBD antitumour antibiotic, anthramycin, was discovered in 1965 (Leimgruber, et al., J. Am. Chem. Soc., 87, 5793-5795 (1965); Leimgruber, et al., J. Am. Chem. Soc., 87, 5791-5793 (1965)). Since then, a number of naturally occurring PBDs have been reported, and over 10 synthetic routes have been developed to a variety of analogues (Thurston, et al., Chem. Rev. 1994, 433-465 (1994)). Family members include abbeymycin (Hochlowski, et al., J. Antibiotics, 40, 145-148 (1987)), chicamycin (Konishi, et al., J. Antibiotics, 37, 200-206 (1984)), DC-81 (Japanese Patent 58-180 487; Thurston, et al., Chem. Brit., 26, 767-772 (1990); Bose, et al., Tetrahedron, 48, 751-758 (1992)), mazethramycin (Kuminoto, et al., J. Antibiotics, 33, 665-667 (1980)), neothramycins A and B (Takeuchi, et al., J. Antibiotics, 29, 93-96 (1976)), porothramycin (Tsunakawa, et al., J. Antibiotics, 41, 1366-1373 (1988)), prothracarcin (Shimizu, et al, J. Antibiotics, 29, 2492-2503 (1982); Langley and Thurston, J. Org. Chem., 52, 91-97 (1987)), sibanomicin (DC-102)(Hara, et al., J. Antibiotics, 41, 702-704 (1988); Itoh, et al., J. Antibiotics, 41, 1281-1284 (1988)), sibiromycin (Leber, et al., J. Am. Chem. Soc., 110, 2992-2993 (1988)) and tomamycin (Arima, et al., J. Antibiotics, 25, 437-444 (1972)). PBDs are of the general structure:

They differ in the number, type and position of substituents, in both their aromatic A rings and pyrrolo C rings, and in the degree of saturation of the C ring. In the B-ring there is either an imine (N═C), a carbinolamine (NH—CH(OH)), or a carbinolamine methyl ether (NH—CH(OMe)) at the N10-C11 position which is the electrophilic centre responsible for alkylating DNA. All of the known natural products have an (S)-configuration at the chiral C11a position which provides them with a right-handed twist when viewed from the C ring towards the A ring. This gives them the appropriate three-dimensional shape for isohelicity with the minor groove of B-form DNA, leading to a snug fit at the binding site (Kohn, In Antibiotics III. Springer-Verlag, New York, pp. 3-11 (1975); Hurley and Needham-VanDevanter, Acc. Chem. Res., 19, 230-237 (1986)). Their ability to form an adduct in the minor groove, enables them to interfere with DNA processing, hence their use as antitumour agents.
It has been previously disclosed that the biological activity of this molecules can be potentiated by joining two PBD units together through their C8/C′-hydroxyl functionalities via a flexible alkylene linker (Bose, D. S., et al., J. Am. Chem. Soc., 114, 4939-4941 (1992); Thurston, D. E., et al., J. Org. Chem., 61, 8141-8147 (1996)). The PBD dimers are thought to form sequence-selective DNA lesions such as the palindromic 5′-Pu-GATC-Py-3′ interstrand cross-link (Smellie, M., et al., Biochemistry, 42, 8232-8239 (2003); Martin, C., et al., Biochemistry, 44, 4135-4147) which is thought to be mainly responsible for their biological activity.
One example of a PBD dimer is SG2000 (SJG-136):

(Gregson, S., et al., J. Med. Chem., 44, 737-748 (2001); Alley, M. C., et al., Cancer Research, 64, 6700-6706 (2004); Hartley, J. A., et al., Cancer Research, 64, 6693-6699 (2004)) which has been involved in clinical trials as a standalone agent, for example, NCT02034227 investigating its use in treating Acute Myeloid Leukemia and Chronic Lymphocytic Leukemia (see: https://www.clinicaltrials.gov/ct2/show/NCT02034227).
Dimeric PBD compounds bearing C2 aryl substituents, such as SG2202 (ZC-207), are disclosed in WO 2005/085251:

and in WO2006/111759, bisulphites of such PBD compounds, for example SG2285 (ZC-423):

These compounds have been shown to be highly useful cytotoxic agents (Howard, P. W., et al., Bioorg. Med. Chem. (2009), doi: 10.1016/j.bmcl.2009.09.012).
In an impact study submitted to the 2014 Research Excellence Framework (REF) in the United Kingdom by University College London (available at http://impact.ref.ac.uk/casestudies2/refservice.svc/GetCaseStudyPDF/35393, it was commented that:
“The next generation of PBD dimers, which are more potent than SG2000, have been developed, including SG2057 and SG2202. They exhibit picomolar/sub-picomolar activity against a range of human tumour cell lines and demonstrate curative activity in human tumour xenograft models.” making reference to:
Hartley J A, et al., DNA interstrand cross-linking and in vivo antitumor activity of the extended pyrrolo[2,1-c][1,4]benzodiazepine dimer SG2057. Invest New Drugs. 2012 June; 30(3):950-8. http://dx.doi.org/10.1007/s10637-011-9647-7 (herein after “Hartley et al (2012)”)
and:
“The ability to generate such cytotoxic molecules that display exquisite potency suggested a potential role in strategies aimed at targeting and releasing highly cytotoxic agents directly at a tumour site. An example is as the ‘warhead’ component of an antibody drug conjugate (ADC). The fully synthetic PBD dimers are ideally suited for the role of warhead in an ADC approach.”
The Hartley et al (2012) paper comments in its summary that “SG2057 is therefore a highly active antitumour agent, with more potent in vitro activity and superior in vivo activity to SG2000, warranting further development”.
SG2057 has the structure:

Antibody drug conjugates using SG2057 as a warhead were first disclosed in WO 2011/130598. For example, claim 54 of this application includes the formula:

wherein n is from 1 to 24, more preferably 4 to 8. The following drug linkers were exemplified: n=4, 15c; n=8, 15d; n=24, 15e.
Claim 54 of this application also includes the formula:

wherein n is from 1 to 24, more preferably 4 to 8. The following drug linkers were exemplified: n=8, 58; n=24, 61.
WO 2011/130598 also discloses antibody-drug conjugates including these drug linkers, for example 110 (antiSteap1-15d), example 114 (tastuzumab-15d) and example 115 (tastuzumab-58).
WO 2013/055987 discloses the drug linkers 14 and 22:

and their use in antibody-drug conjugates.